555 Timer Help

They call it the trigger for a reason. Tying it to Vcc means your timer will never start.

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

Two things:

1. Pin 5 can be left unconnected or connect a small capacitor to it going to ground.
2. Pin 2 is the trigger, as bhankiii pointed out. To get the output to go high, this pin must be pulled low to at least 1/3 of the supply voltage, in your case, 4 volts. After pulling it low, you must bring it back up high (to Vcc).

If you're using this as a power-on reset, put another resistor-capacitor on pin 2. The resistor goes to Vcc and pin 2, and the capacitor goes to pin 2 and ground. Use smaller values than what you used for the timing.

What this does is to hold pin 2 at a low voltage during power up, allowing the 555 to start its timing. Before the delay ends, the voltage at pin 2 must be above 1/3 Vcc.

If you're switching this thing off frequently, put a diode across the capacitor on pin 2 (cathode side to pin 2) to discharge it faster when power is removed.

The 555 is a clumsy way to do time delays. Get yourself a hex CMOS Schmidt trigger inverter like a 74C14 or 40106 and you now have up to 6 timers, one-shots or any number of other functions can be configured. Oscillators, flip-flops too! And all for microamps. True you don't have the high current output but sometimes you don't need that. Better yet get a Microchip six pin SO-23 package microcontroller and generate whatever feature you want.

K.I.S.S

Check this link out.

http://home.cogeco.ca/~rpaisley4/LM555.html

The 555 IC is really not a practical way of doing things like your delay (especially when you are expecting fractions of a second for accuracy).

I remember using these in school because they were part of an old practice lab, and really after I got on the field, my lab partner got a job in NASA in FL. The first thing we laughed about was the use of the 555, and why they teased him.

It seems that the RC constants in these IC's vary greatly between chip and chip, even in the same batch of chips. You already have the tolerances on other components to worry about (your resistors and capacitors), so if you want a system that you can depend on, and a system that you can reproduce without variations, get rid of the 555. It is a relic for this application.

To be fair, you can do well with it once you focus on one chip and have good instruments to mess with it, but it is clumsy, and it should never be used on commercial systems that need to have precision and a long life (because you'd be using sockets to replace old chips, and the new chip will have different RC constants built in).

Good luck on your project!

For high precision, no, you never use a 555 timer. For anything else, nothing beats its low cost, availability and ease of use. Others may argue otherwise but I've used 555s in microprocessor based applications and have been quite happy with the results, including the customers. I met one of them a few weeks ago and asked about my project with him and he says it's still working 20 years later.

If you want precision and higher frequencies, then use a crystal-based circuit (which can be done with the 555, by the way).

I still see the 555 in some commercial products so it's not a dinosaur yet.

I just want to point out....

the OP... NEEDS to use a 555 timer.. it is for a school lab project (at least, that's what the phrasing leads me to believe). While all the other suggestions of (more accurate/repeatable) alternatives, are all well & good.. they do the OP absolutely no good :P

now, with that being said.. i can't remember anything about 555's, so i'm not going to contribute on how you hook it up.. aside from the obvious (and already mentioned)... the "trigger".. is just that... a trigger... connecting it to VCC will never allow the timing sequence to start... something has to tell the system to "go" afterall!

Check out http://cache.national.com/ds/LM/LM555.pdf

This is a datasheet for the LM555. If you want the thing to trigger itself as a multivibrator you have to have a cap installed like Figure 4 otherwise don't connect the trigger to the vcc.

I am sure you can get your needed help by searching 555 timer on the internet.

Regards